Dutch forensic investigators told armed separatists guarding train cars full of bodies from the downed Malaysia Airlines jet that the train must be allowed to leave within hours.

The experts from the Dutch National Forensic Investigations Team - which specialises in victim recovery and identification - also pressed for rebels to seal the train cars parked in the rebel-held town of Torez.

Journalists at the site said the smell of decay was overwhelming Monday and many with the inspectors wore masks or pressed cloths to the faces on the warm summer day. A train engineer said that a power outage had hit the cars' refrigeration system overnight and it was not immediately clear why. The cooling system was back up and running early Monday, he said.

The fumbling efforts to recover remains and secure evidence at the crash site have fanned international outrage. Four days after the jetliner was shot out of the sky, international investigators still have had only limited access to the crash site, hindered by pro-Russia fighters who control the verdant territory in eastern Ukraine.

Of the 298 victims killed, 192 were Dutch and another was Dutch-American.

The investigators led by Peter Van Vilet, of the Dutch LTFO forensic office, climbed aboard to inspect the wagons, surrounded by armed rebels standing nearby.

"I have watched the train and the wagons and I think the storeage of the bodies is good of quality," Van Vilet said.

"We got the promise the train is going," he said, adding he did not know when.

The investigators stood for a moment with heads bowed and hands clasped before ascending the wagon.

Australia, meanwhile, pressed for a UN resolution calling for uninhibited access to the rebel-controlled crash site. It also asks for the full cooperation of all countries in the region, including Russia.

Emergency workers retrieved 21 more bodies on Monday from the charred crash site and surrounding fields near the eastern village of Hrabove. Ukraine's prime minister said 272 bodies have been recovered of the 298 people killed aboard the plane.

In the government-controlled eastern city of Kharkiv, another team of experts came to the government crisis centre focused on the crash. Prime Minister Arseniy Yatsenyuk said the team included 23 Dutch, two Germans, two from the United States, one from the United Kingdom, and three representatives from the Australian embassy.

The United States presented what it called "powerful" evidence on Sunday that the rebels shot down the Boeing 777 with a Russian surface-to-air missile.

That evidence included video of a rocket launcher, one surface-to-air missile missing, leaving the likely launch site; imagery showing the firing; phone calls claiming credit for the strike and phone recordings said to reveal a cover-up at the crash site.

"A buildup of extraordinary circumstantial evidence ... it's powerful here," said US Secretary of State John Kerry.

"Russia is supporting these separatists. Russia is arming these separatists. Russia is training these separatists," he added.

The head of counter-intelligence for Ukraine's SBU security service, Vitaliy Najda, said on Saturday that the Buk launchers came from Russia and called on Russia to supply the names and ranks of the service personnel "who brought about the launch of the missile" so they could be questioned by investigators.

He said the rebels could not have operated the sophisticated weapon without Russian help. Mr Nayda did not provide specific evidence for his claim.